A qubit circuit is a circuit wherein two quantum states can coexist in quantum superposition states and that supports computational use of the two quantum states as representations of logical 0 and 1 states. Various examples of qubit circuits are known under the names of Cooper pair box, Transmon, Quantronium, Fluxonium, etc. US2009015317 discloses examples of qubit circuits. The disclosure of the qubit circuit in this document is incorporated herein by reference. In most instances the qubit circuit comprises a superconducting resonant RF circuit with a resonance frequency in the microwave range, for example in a range of 1-10 Gigahertz and preferably a Josephson junction in this structure. In quantum computation circuits qubit circuits are used as elements for processing and storing superimposed information.
It is known to transmit RF pulses to qubit circuits to control state changes and/or measure states. An RF pulse generation is used to apply RF pulses to the input of such a qubit circuit, for example via a microwave transmission line that is capacitively coupled to the qubit circuit. For example, the transmission may comprise using the RF pulse to generate a superimposed electromagnetic field on a field in part of the superconducting resonant RF circuit. The frequency of the RF signal in the pulse is related to the resonance frequency of the qubit and may be in the Gigahertz range (although it need not be equal to that resonance frequency). The state change of the qubit depends on the shape of the pulse, its amplitude and the phase of the undulations in the RF pulse (the undulations may correspond to RF oscillations upon which the RF pulse has been modulated). Moreover, pulse shapes of in-phase and quadrature components of the RF pulse may to be controlled to avoid undesired state changes.
A series of successive RF pulses may be used to perform successive state changes. By setting the amplitudes and relative phase of RF pulses in such a series, a selectable series of operations may be performed on the qubit. A control system may be used that stores definitions of sets of such parameters for predefined RF pulses and a “program” that indicates which of these predefined RF pulses must be transmitted in the series. In a qubit system with a plurality of qubits, a control system selects the amplitude and phase of the RF signal in an RF pulse for different qubits dependent on the desired state changes.
Manufacturing tolerance may detrimentally affect the usefulness of qubit circuits in quantum computation circuits. Manufacturing tolerance can result in inaccuracy of state changes and insufficient effectiveness of suppression of undesired state transitions. Noise can reduce the coherence time of the qubit. To compensate for manufacturing tolerances, slightly different amplitude and/or phase settings may be needed for RF pulses to different individual qubits to affect the same state changes.
It is known to use independent RF pulse generators in a multi-qubit system to generate the RF pulses with controlled amplitude and phase for the individual qubits. In this way, calibration of the RF pulse generators can be used to account for the effects of manufacturing tolerances. Calibration may be performed for example by applying one or more RF pulses to each qubit and measuring the effect of the RF pulses. RF pulses with different amplitudes and/or phase may be applied to each qubit during calibration to measure a responses of individual qubits as a function of amplitudes and/or phase and the amplitudes and/or phase corresponding to a predetermined desired response may be used to control the setting of the adapters. Moreover, with independent RF pulse generators arbitrary combinations of pulses can be applied to different qubits simultaneously, which makes it possible to make optimum use of the coherence time. However, this solution requires that a large number of RF pulse generators be used in a qubit system with a large number of qubits. With increasing numbers of RF pulse generators this will to high equipment costs and also to increasingly significant heat leak, because the RF pulse generators conventionally operate at room temperature and transmission lines have to be provided to the superconducting qubits that operate at very low temperature (e.g. cryogenic temperatures).
US 20090014714 discloses a control system architecture for quantum computing. The document addresses the problem of multiplexing signals at low temperature. The document uses a 2-d matrix of quantum circuits such as qubit circuits. The matrix has column conductors, each coupled to the qubits in a respective column of the matrix, to apply a bias signal to the qubits circuits in the column. Furthermore the matrix has row conductors, each coupled to the qubits circuits in a respective row of the matrix, for applying microwave pulses to the qubits circuits in the row. In this way, if there are M columns and N rows, i.e. N*M qubit circuits, N transmission lines for control signals suffice to address the control signals to all of the N*M qubit circuits individually on a time division multiplexing basis. However, resonance frequency discrepancies between different qubits can detrimentally affect the performance of such a qubit matrix. Furthermore, compared with the solution using individual RF pulse generators for individual qubits, the number of RF pulses that can be applied simultaneously is significantly reduced, so that some tasks that could be executed with the same number of qubits using individual RF pulse generators cannot be executed with such a matrix within a time distance that is smaller than the coherence time.